(1) Field of the Invention
The present invention relates to an optical recording medium on which recording can be made, when irradiated with light having a wave length in a near infrared region.
(2) Description of the Prior Art
A heat-sensitive recording system is a direct recording system which does not require any development and fixing, and therefore it is excellent in operation and maintenance. For this reason, the heat-sensitive recording system is widely utilized in facsimiles, printers and the like.
In this system, however, the recording is thermally made by bringing a thermal head or an exothermic IC pen into direct contact with a heat-sensitive recording paper, and therefore melted colored substances adhere to the thermal head or the exothermic IC pen, so that troubles such as dregs adhesion and sticking take place, which brings about record obstruction and impairs record quality inconveniently.
In particular, when a line is depicted continuously in a recording direction as in the case of a plotter printer, it is impossible to avoid the trouble of the dregs adhesion.
Furthermore, when the recording is made by the thermal head, it is difficult to more heighten an image resolution of 8 dots/mm which is now employed.
Thus, as techniques by which troubles such as the dregs adhesion and the sticking are solved and by which the resolution is more improved, some non-contact recording systems using light have been suggested.
Japanese Patent Laid-open Publication No. 209594/1983 discloses an optical recording medium prepared by laminating at least one set of a near infrared absorbent layer having an absorption wave length in a near infrared region of 0.8 to 2 .mu.m and a heat-sensitive color-developing material layer onto a substrate, and Japanese Patent Laid-open Publication No. 9449/1983 discloses a recording medium prepared by superposing, on a base material, a layer containing one or more kinds of heat-sensitive materials and a layer containing one or more kinds of near infrared absorbents comprising compounds having a maximum absorption wave length in near infrared rays of 0.7 to 3 .mu.m.
These publications disclose the procedure of laminating or superposing the near infrared absorbent and the heat-sensitive color-developing material on the substrate or the base material. That is, the near infrared absorbent is mixed with the heat-sensitive color-developing material and the resulting mixture is then applied onto the substrate or the base material, or alternatively the heat-sensitive color-developing material is first applied on the substrate or the base material, and the near infrared absorbent is then applied on the heat-sensitive color-developing material layer.
Furthermore, in the above-mentioned publications, there are disclosed dyestuffs such as cyanine dyestuffs, thiol nickel complexes and squalilium as the near infrared absorbent having the absorption wave length in a near infrared region of 0.8 to 2 .mu.m or 0.7 to 3 .mu.m.
In addition, as enumerated in "Near Infrared Absorption Dyestuffs", Chemical Industry, Vol. 43, May 1986, other dyestuffs are known which are, for example, nitroso compounds and their metal complexes, polymethine dyestuffs (cyanine dyestuffs), complexes of thiols and cobalt or palladium, phthalocyanine dyestuffs, triallylmethane dyestuffs, immonium dyestuffs, diimmonium dyestuffs and naphthoquinone dyestuffs.
As disclosed in the above-mentioned laid-open publications, the near infrared absorbent and the heat-sensitive color-developing material are applied on the substrate or the base material. That is, these materials are mixed and the resulting mixture is then applied onto the substrate or the base material, or alternatively when the heat-sensitive color-developing material is first applied on the substrate or the base material, and the near infrared absorbent is then applied on this material layer. However, in the above-mentioned mixing step, a desensitization phenomenon occurs, and color development performance declines. Moreover, when the near infrared absorbent is applied onto the material layer, a ground color deteriorates inconveniently. These problems prevent putting the optical recording medium into practical use.